When horrendous crimes are committed in WA, it's not long before the on-call phone of the major crime squad rings.

A detective at home with his family answered. A full-scale crime investigation swung into action. Family time would have to wait.

It was the first of too many calls to that phone this week, as police investigated the deaths of five people in just four days.

The tragic series that has left families shattered and stretched police resources is sadly not unprecedented.

In May, the major crime squad investigated six suspected murders in a fortnight.

As each call came this week, some in the middle of the night, more officers were called away from their families to ensure rapid responses to the terrible crimes.

Det-Supt Anthony Lee said the first hours were critical for collecting crucial evidence and teams worked around the clock to identify those responsible.

The personal cost of the violence was not lost on investigators but they concentrated on remaining professional.

"The community relies on us to remain objective in emotional times," Det-Supt Lee said.

The major crime squad are not the only ones called on.

They rely on support from other sections, including forensics officers who can spend days at crime scenes gathering evidence, other specialist units, local detectives and uniformed officers. Other government departments also provide expertise.

Just hours after police began investigating the deaths of Maureen and Tamara Horstman another call came in.

On Monday at 1am, 50-year-old Brian Keenan had been fatally stabbed outside his Balga home.

Twenty-four hours later, officers were again woken about 1am and asked to head north to Kalbarri where a 33-year-old man had been fatally stabbed.

Within hours, detectives were questioning people over the two stabbings. In each case, those in custody had known the victims.

But the Warwick killings were mysterious. Suspicions initially fell on Tamara's brother but he was soon cleared.

Homicide detectives began to investigate the theory the women were killed during a burglary.

Less than 48 hours after they were found dead, police had identified and arrested a teenage suspect.

They have not revealed what led them to the 19-year-old, who was charged with the terrible crime.

It is believed the discovery in Mirrabooka of Miss Horstman's abandoned stolen car, spotted by a resident after police appealed for help to track it down, provided clues.

Det-Supt Lee said community information could be critical to the success of an investigation.

"Without the help of witnesses or people who have information about an incident, many offences would go unsolved," he said.

After swooping on the Warwick suspect, detectives had little respite.

A few hours later, some of the same officers were woken by news of another violent death - a 20-year-old had been fatally slashed in the neck on a beach early on New Year's Day after a New Year's Eve party.

The five homicides came just days after the squad took over the inquiry into a one-punch attack on Irishman Thomas Keaney in Northbridge when it became clear he would not survive.

That case came to them on Christmas Day, just three days after they were called to the fatal bashing of a teenager in a Balga park.

The major crime squad went to 96 incidents last year.

The call-outs led to investigations into the murders of 42 West Australians and suspected homicides of two others.

Det-Supt Lee said the squad often handled several murder cases simultaneously. Set inquiry procedures enabled officers to be shifted quickly between investigations depending on where resources were needed.

Despite the high number of cases - up from 38 confirmed and suspected homicides in 2012 - just one inquiry from last year is unsolved.

Helen Rocha, a mother of two, vanished after leaving her home with a friend in September.

Police suspect she has been murdered but her body has not been found and no one has been charged over her disappearance.